The present invention relates to liquid filtration processes and systems and, more particularly, to filtration processes and apparatus for removing both large, bulky and small, compactible solid contaminants from liquids used for cooling and/or lubricating purposes in metal machining operations, and the like.
It is a common practice to employ filtration systems in connection with metal machining operations using liquid coolants and lubricants. As the liquid carries away metal chips, turnings, and other materials from the work areas it must be clarified before being again pumped through the system. A basic form of apparatus used in such filtration processes comprises a settling tank with a porous filter material or screen separating an upper compartment, to which the contaminated liquid is delivered, from a lower compartment, into which the filtered or clean liquid drains. The filtration efficiency increases as small contaminants build up to form a bed on the surface of the filter since these contaminants also serve to block the flow of other solid materials in the liquid. However, as efficiency increases, the rate of flow of liquid from the contaminated to the clean compartment and the pressure within the clean compartment correspondingly decrease.
A drag conveyor having transverse flights contacting the upper surface of the filter element is commonly used to remove some of the collected contaminants when the rate of flow, usually as evidenced by the pressure differential between the upper and lower compartments, reaches a predetermined level. In typical installations, the conveyor is stationary until the sensing apparatus indicates that flow through the filter has decreased below the set point, at which time the drive mechanism is actuated to move the conveyor and thus remove contaminants from the filter element surface. When sufficient contaminants have been removed to restore the flow rate to a desired value, the conveyor is stopped and remains stationary until flow rate again decreases below the set point.
While such systems are effective to maintain automatically the flow rate of liquid from upper to lower compartment between predetermined maximum and minimum values, it is apparent that the flow rate and filtration efficiency will constantly fluctuate between these values. Thus, it is difficult to maintain a constant rate of withdrawal of clean liquid from the lower compartment and a constant degree of clarity of the filtered liquid thus requiring a second tank to maintain the balance of the flow. Another problem results from the fact that while certain of the contaminants are relatively small and compactible, thereby collecting on the upper surface of the filter element to form the so-called sludge bed, others are large and bulky. The latter are commonly curled metal turnings, having large bulk compared to their density and creating little resistance to the flow of liquid therethrough. Thus, the upper compartment may contain a large amount of turnings without decreasing flow rate (i.e., increasing pressure differential) to the point where the conveyor is indexed to remove the turnings. In fact, since filtration systems of this type may be rendered virtually inoperative due to the buildup of turnings in the upper compartment, a separate settling tank is commonly provided for removal of turnings and other such bulky contaminants prior to delivery of the liquid to the filtration tank. Obviously, this requires additional handling of the liquid, more space, more equipment and more time to effect purification of the liquid than a process which can be carried out in a single tank.
It is a principal object of the present invention to provide a process whereby both large, bulky and small, compactible contaminants are removed from a liquid in an automatic manner which maintains a substantially constant degree of liquid clarity.
A further object is to provide a process and apparatus for purifying and recycling liquid to a metal machining operation in a manner which minimizes the required energy, space, time and equipment.
Another object is to provide liquid filtration apparatus of the foregoing type having a desired, substantially constant filtration efficiency by maintaining a constant pressure differential across the filter and bed of accumulated contaminants.
In a more general sense, the object of the invention is to provide novel and improved filtration processes and apparatus for purifying liquid coolants used in metal working operations.
Other objects will in part be obvious and will in part appear hereinafter.